1. Field of the Invention
The present invention relates to a surface mount quartz crystal oscillator, and more particularly, to a surface mount crystal oscillator which facilitates a reduction in size.
2. Description of the Related Art
A surface mount crystal oscillator which accommodates a quartz crystal unit and an oscillator circuit using the crystal unit in a surface mount package has been widely used, particularly for portable electronic devices as a frequency or a time reference source because of its small size and light weight. In recent years, an increasing reduction in size has been rapidly advanced in a variety of portable devices represented by portable telephones, causing an associated reduction in size of surface mount crystal oscillators.
A conventional surface mount crystal oscillator illustrated in FIG. 1A employs package body 1 formed with a recess which receives IC (integrated circuit) chip 2 and quartz crystal blank 3, and cover 4 which is placed over the recess for closing the same to hermetically encapsulate IC chip 2 and crystal blank 3 within package body 1.
Steps are formed in the inner wall of the recess of package body 1. Package body 1 is made of a three-layered laminated ceramics comprising a bottom plate, a first frame wall corresponding to the steps and a second frame wall corresponding to a portion of the recess above the steps. The frame wall refers to a frame-shaped member which has an opening corresponding to the recess. IC terminals, not shown, are formed at appropriate positions on the bottom of the recess for use in connection to IC chip 2. Also, a pair of crystal terminals, not shown, are formed on one step, as shown, in the recess of package body 1 for use in connection to crystal blank 3. Further, conductive paths (i.e., wiring pattern), not shown, are formed in package body 1 for connecting the IC terminals to the crystal terminals.
IC chip 2 has an oscillator circuit integrated therein which is connected to crystal blank 3 to make up an oscillator, and is connected to the IC terminals using bumps 5, for example, by ultrasonic thermo-compression bonding hence secured to the inner bottom surface of the recess of package body 1.
Crystal blank 3, which is comprised, for example, of an AT-cut quartz crystal blank, is substantially rectangular in shape as illustrated in FIG. 1B, and is formed with excitation electrodes 6 in central regions of both main surfaces, respectively. Extending electrodes 7a, 7b are extended from a pair of excitation electrodes 6 toward both ends on one shorter side of crystal blank 3, respectively. The leading end 24 of each extending electrode 7a, 7b is folded over onto the opposite main surface of crystal blank 3. Crystal blank 3 is secured with conductive adhesive 8 at both sides of one end thereof, on which extending electrodes 7a, 7b are extended, on the step, on which the crystal terminals are formed. In this event, crystal blank 3 is secured while the other end of crystal blank 3 is carried on the other step, as shown. In this way, crystal blank 3 is connected between an input terminal and an output terminal of an oscillation amplifier within IC chip 2 through the crystal terminals, conductive paths routed on package body 1, and the IC terminals.
At four corners on the bottom of package body 1, mounting electrodes (not shown) are formed for use in surface-mounting the crystal oscillator on a wiring board. The mounting electrodes also extend off outer side surfaces of package body 1. The mounting electrodes, which will serve as a power supply terminal, a ground terminal, an output terminal, and the like of the crystal oscillator, are electrically connected to IC chip 2 through conductive paths, not shown, and the IC terminals.
In the foregoing crystal oscillator, however, crystal blank 3 has one end held on one step within the recess of package body 1, and the other end carried on the other step, so that the resulting crystal oscillator has a problem of large outer dimensions. With the use of an AT-cut quartz crystal blank which has a thickness shear vibration mode, the crystal blank itself can be fairly reduced in size. However, in the structure described above, crystal blank 3 must be mounted to straddle IC chip 2, so that crystal blank 3 must have a horizontal length, as shown, larger than the horizontal length, as shown, of IC chip 2. Actually, slow progress of reduction in size of IC chip 2 eventually results in difficulties in reduction in size of a crystal oscillator.
To solve the foregoing problem, Japanese Patent Laid-open Application No. 2002-280835 (JP, P2002-280835A) proposes a crystal oscillator, as illustrated in FIG. 2, in which a crystal blank has one end secured on a step in a recess of package body 1 and the other end carried on a spacer disposed on IC chip 2. The crystal oscillator thus constructed can be reduced in size because the other step is eliminated within a pair of steps in the recess of package body 1.
However, even in the structure illustrated in FIG. 2, a step required for disposing the crystal terminals causes the crystal oscillator to have an outer planar size much larger than the size of IC chip 2, thus inhibiting the reduction in size of the crystal oscillator.